Congenital dyserythropoietic anemia type-II (CDAII) is an autosomal recessive disease resulting from a defect in terminal erythroid maturation, characterized by moderate anemia and increased bone marrow (BM) bi/multi- nucleated erythroblasts. CDAII results from mutations in SEC23B, one of two closely related mammalian SEC23 paralogs (SEC23A and SEC23B). SEC23 is a core component of coat protein complex-II (COPII) vesicles, which transport secretory protein cargos from the Endoplasmic Reticulum (ER) to the Golgi apparatus. BM transplantation cures CDAII, suggesting that the pathologic defect is restricted to the hematopoietic compartment. Despite the identification of the genetic defect underlying CDAII, the mechanism by which SEC23B deficiency results in CDAII remains unknown. We previously showed that, in contrast to humans, SEC23B-deficient mice die perinatally, exhibiting massive pancreatic degeneration. We also generated mice with SEC23B deficiency restricted to the hematopoietic compartment and showed that these mice lack anemia and other CDAII characteristics. In recent preliminary results, we genetically engineered the Sec23a cDNA into the genomic locus of Sec23b in mice. Mice expressing SEC23A under the normal temporal and tissue specific patterns of SEC23B appear healthy and survive past weaning. This surprising observation suggests that the two Sec23 paralogs are interchangeable at the level of protein function. Additional preliminary data demonstrate that the SEC23B/SEC23A expression ratio is higher in human BM compared to pancreas, with the reverse pattern observed in mice. Taken together, these data suggest that the tissue-specific expression patterns of the Sec23 paralogs have shifted during evolution between mouse and human and that the absence of CDAII in SEC23B-deficient mice is due to compensation from SEC23A, the latter accounting for a relatively high fraction of total SEC23 in the hematopoietic compartment of these animals (in contrast to humans). The overall objective of this proposal is to study the functional evolution of the two SEC23 paralogs and to characterize the molecular pathogenesis of CDAII. First, I will determine the extent to which the highly similar SEC23 paralogs functionally compensate for one another in-vivo. I will fully characterize SEC23B deficient mice expressing SEC23A under the endogenous Sec23b regulatory elements and assess for subtle abnormalities in pancreas function. If these mice are anatomically normal and exhibit normal survival, growth, and fertility, this will demonstrate that SEC23A and SEC23B are interchangeable at the protein level when expressed in the appropriate tissues and at the appropriate times and levels. Second, based on SEC23A and SEC23B expression data, and on our preliminary work demonstrating that the 2 SEC23 paralogs overlap in function, we hypothesize that mice with hematopoietic SEC23A (or combined SEC23A/SEC23B) deficiency will develop CDAII. We will generate and characterize these mice, and expect to generate a murine model faithfully reproducing the human CDAII phenotype. Third, based on the ER-to-Golgi transport defect in this disease, we hypothesize that CDAII results from the impaired trafficking of one or more key cargo proteins that depend on SEC23B for ER-exit, and therefore for secretion to the RBC plasma membrane. We will perform quantitative proteomic techniques to identify the critical cargo(s) that depend on SEC23B for exit from the ER in human erythroid cells. We will validate the putative cargo(s) for their roles in CDAII pathophysiology. These studies have important implications for understanding the evolutionary functions of the paralogous SEC23 genes, and for improving our fundamental understanding of the complex mechanisms by which cargos are sorted in COPII vesicles. The results from this project may lead to the identification of new therapeutic targets as well as model systems in which to test these targets. Cargos that depend on SEC23B for secretion during RBC development may represent novel targets for CDAII therapy, which are also expected to translate to other anemias due to defects of terminal erythroid maturation. We also expect to generate a mouse model faithfully reproducing human CDAII, which may be used in the future to test novel therapies. Determining the functional overlap of SEC23A and SEC23B is critical because a shared role of these paralogs would suggest that therapies that increase the expression of either paralog in erythroid cells might be effective in CDAII.